A particular class of thin-film photovoltaic devices has an absorber layer formed of a group I-III-VI semiconductor, also referred to as a chalkopyrite semiconductor. Such a semiconductor is generally of the copper indium diselenide (“CIS”) type, wherein this expression is to be understood such that indium can be partly or fully replaced by gallium and/or aluminium, and selenium can be partly or fully replaced by sulphur. CIS type semiconductors include those characterized by the formula CuInxGayAl(1-x-y)SezS(2-z), wherein x+y≦1 and z≦2. Special cases of a CIS type layer are e.g. also denoted as CIGS or CIGSS. The CIS type layer can further comprise a low concentration, trace, or a doping concentration of one or more further elements or compounds, in particular alkali such as sodium, potassium, rubidium, cesium, and/or francium, or alkali compounds. The concentration of such further constituents is typically 5 wt % or less, preferably 3 wt % or less.
The CIS type layer is typically arranged on a back electrode of a metal film such as Mo which is supported by a substrate, typically soda-lime glass, but other substrates are possible as well. Preferably a diffusion barrier layer such as of silicon nitride or silicon oxide is arranged between the glass substrate and the back electrode.
The thin-film photovoltaic device is typically completed by depositing a buffer layer of cadmium sulphide on the CIS type layer, followed by one or more layers of a transparent metal oxide, often zinc oxide on which metal contacts are arranged. The top layer of the metal oxide is arranged to be an n-type semiconductor, e.g. by doping ZnO with boron or aluminium.
In the manufacture of photovoltaic devices such as solar cells and solar modules, it is desired to optimise a number of parameters such as efficiency, open circuit voltage, short circuit current, fill factor, and at the same time one wishes to fulfill certain requirements like long-term stability and resistance to environmental influences such as temperature and moisture. A further goal is to minimize or eliminate the use of cadmium.
U.S. Pat. No. 4,612,411 discloses a thin-film photovoltaic device comprising subsequently a layer of p-type copper indium diselenide semiconductor; a layer of high-resistivity zinc oxide, and a layer of low-resistivity zinc oxide, wherein both zinc oxide layers are deposited by chemical vapour deposition (“CVD”). No buffer layer of CdS is present. The high-resistivity zinc oxide can further comprise sulphur. In a CVD process, the substrate is exposed to a chemically reactive vapour composition, wherein the reaction taking place on the substrate surface produces the desired deposit as a film. Although good electrical performance can be achieved with such a cell, it has been reported in International Patent Application publication No. WO97/36334 that modules with CVD deposited zinc oxide show relatively large degradation in a so-called damp-heat test, and that this problem can be solved by arranging a further barrier layer on top of the ZnO.
European patent application publication No. EP 0 604 801 A2 discloses a thin film photovoltaic device comprising a semiconductor film onto which an insulating zinc oxide film has been chemically deposited from solution, and thereupon a CVD n-type zinc oxide.
In a presently preferred process as it is described in J. Palm, V. Probst and F. H. Karg, “Second generation CIS solar modules” Solar Energy, vol. 77, p. 757-765, 2004, a CdS buffer layer is employed, onto which a thin layer (ca. 75 nm) of intrinsic ZnO is deposited by RF magnetron sputtering from a ceramic ZnO target. Subsequently, a 600-800 nm thick n-type ZnO layer is deposited by DC magnetron sputtering from a ZnO-Al2O3 target. U.S. Pat. No. 5,871,630 discloses another process in which a CdS layer is deposited, followed by RF sputtering of two zinc oxide layers.
Another deposition method is discussed in the article “High efficiency chalkopyrite solar cells with ILGAR-ZnO WEL-device characteristics subject to the WEL composition, M. Bär et al., Conference record of the 29th IEEE Photovoltaics specialists conference, vol 29, 10 May 2002. ZnO is deposited using Ion Layer Gas Reaction (ILGAR), which method includes first dipping the substrate in a Zn2+ precursor solution, drying, and chemical conversion of the solid precursor in a gaseous atmosphere.
Research has been conducted to find alternatives for CdS for use as buffer layer on top of a CIS type absorber layer. German patent specification DE 44 40 878 C1 discloses a CIS type thin film solar cell with a cadmium free buffer layer including a ternary compound of a metal component, sulphur, and oxygen, wherein the metal component is indium or tin, and with a ZnO window layer on top. The articles “Performance improvement of CIGS based modules by depositing high-quality Ga-doped ZnO windows with magnetron sputtering”, Baosheng Sang et al., Solar Energy Materials & Solar Cells 67 (2001) 237-245; “18% Efficiency Cd-free Cu(In,Ga)Se2 thin-film solar cells using chemical bath deposition (CBD)-ZnS buffer layers”, Tokio Nakada et al., Jpn. J. Appl. Phys. Vol 41 (2002) pp L165-L167; “Highly efficient Cu(Ga,In)(S,Se)2 thin film solar cells with zinc-compound buffer layers”, A. Ennaoui et al.; Thin Solid Films 431-432 (2003) 335-339; “Atomic layer deposition of Zn(O,S) buffer layers for high efficiency Cu(In,Ga)Se2 solar cells”, Ch. Plazer-Björkman et al., Proc. 3rd World Conference on Photovoltaic Energy Conversion, Osaka, Japan, 2003; Cu(In,Ga)Se2 thin film solar cells with buffer layer alternative to CdS”, R. N. Bhattacharya et al., Solar Energy 77 (2004) 679-683 disclose various such alternative materials including Ga-doped ZnO, ZnS, Zn(O,S), ZnS(O,OH) for buffer layers interposed between the CIS type layer and one or more sputtered ZnO window layers.
The article “Development of Cu(In, Ga)Se2-based thin-film PV modules with a Zn(O,S,OH)x buffer layer”, Katsumi Kushiya, Solar Energy 77 (2004) 717-724, discloses a Zn(O,S,OH)x buffer layer deposited by chemical bath deposition between a Cu(In,Ga)Se2 layer and a boron doped zinc oxide window layer deposited by metal organic chemical vapor deposition.
It is an object of the present invention to provide a photovoltaic device based on a CIS type absorber layer of improved performance.
It is a further object to provide a method for manufacturing such a device.